1. Field of the Invention
The invention relates to a power supply device, and more particularly to a power supply device, which is capable of auto-cutting off a starting loop.
2. Description of the Related Art
Since internal elements of electronic products mostly utilize DC voltage, a power supply device is utilized to transform AC voltage into the DC voltage. When the internal elements of the electronic products receive the DC voltage, the internal elements can normally operate. Generally, power supply devices can be classified as a linear mode power supply device and a switch mode power supply device.
The linear mode power supply device receives and transforms an input voltage to generate an output voltage. Although the linear mode power supply has a simple circuit and high stability, the efficiency of the linear mode power supply is unacceptable when the difference between the input voltage and the output voltage is great. In such a case, the linear mode power supply cannot directly be disposed in the electronic products without cooperating with a switch mode power supply device. To the contrary, the switch mode power supply device has a small size and high transformation efficiency. Thus, the switch mode power supply device is critical to be applied in electronic products.
The switch mode power supply device generally comprises a control circuit. The control circuit receives energy via a start resistor and then executes a power conversion operation. After the control circuit is activated, the control circuit can receive energy via another loop. Thus, the control circuit is not required to receive energy via the start resistor. However, power consumption of the start resistor is still caused because some current continuously flows through the start resistor.
To reduce the power consumption caused by the start resistor, a conventional method is to increase resistance value of the start resistor, but it also increases the start time of the control circuit. However, refusing to increase the resistance value of the start resistor will cause more power consumption than start resistor with high resistance value.